Recently, I was typing in a post and took a tad longer than I thought I would. When I went to preview my post, it asked me my login (again) and then went on to say I do not have any current thread. I lost the entire post, as 'back' does not help either.

The default time-out on your cookie is 15mins or 900secs. This can be modified but I'd rather not do it unless a great number of people is starting to have problems. A good idea when posting is always to use the option preview post because you *can* usually still use your button.

Yes, I must have been at it at least 20 minutes before I decided to 'Preview Reply'. However, I think I did not explain the problem correctly. What happened was that when I hit 'Preview Reply', the LOGIN screen came up. So, I logged in (again!). THEN it tells me there is no current thread (for preview). And I lose all my typing. After that, the 'back' button takes me back to the login screen, which is not useful.

In any case, I must remember the limitation and prepare my post off-line before I actually paste it for the post.

I have had the same problem, but you do not loose your post.
Instead of use buttom back 1 step, use menu option in your Browser Go To. There you will see your Reply. Go there, copy and paste in a new try.